1. Field of the Invention
The present invention relates to an optical amplifier configured by optically cascade-connecting a plurality of amplification units each including a pumping light source and an amplifying optical fiber.
2. Related Background Art
Development of a wavelength division multiplexing (WDM) optical communication system that can perform optical transmission of signal light including one or more signal channels having mutually different wavelengths has actively been promoted. In a photonic network, among others, an optical add/drop multiplexer (OADM) and optical cross-connect (OXC) are provided in a node and thus the number of signal channels included in signal light varies frequently from place to place in a network. Therefore, the number of signal channels in signal light inputted into an optical amplifier provided in such a photonic network may naturally vary. Transient signal light gain variations can be listed as one of important performance evaluation items of an optical amplifier when the number of signal channels of signal light inputted into the optical amplifier varies.
That is, in a photonic network, the power of signal light inputted into an optical amplifier varies resulting from the variation of the number of signal channels. At this point, inside the optical amplifier, a gain of signal light remaining in an amplifying optical fiber varies as signal light power varies. In general, when the input signal light power decreases, a transient gain increase called an overshoot of signal light remaining inside the amplifying optical fiber occurs. When the input signal light power increases, on the other hand, a transient gain decrease called an undershoot occurs.
Both the overshoot and undershoot of gain change are known to adversely affect transmission systems. An overshoot of gain change may cause a receiver to fail due to excessive power input. An undershoot of gain change, on the other hand, may cause a transmission error due to power deterioration out of receivable range. Thus, it becomes necessary to suppress such overshoots and undershoots of gain change.
An optical amplifier generally performs constant gain control by controlling a pumping light power supplied to the amplifying optical fiber from a pumping light source so that a ratio of the input signal light power to the output signal light power reaches a target value while monitoring both the input signal light power and the output signal light power. Suppressing the overshoot and undershoot of gain change by performing pumping light power control at high speed based on the constant gain control was proposed by Haruo Nakaji et al. in “High-speed Automatically Gain Controlled EDFAs”, Technical Report of IEICE, OFT, OFT2001-56, pp. 13-18 (2001).